In the past I used to buy used Supermicro servers off of eBay.  I had good luck with the.  I've moved past the stage of cobbling things together and have been buying Synology DiskStations more recently but for what you are wanting to do I'd probably go the eBay/Supermicro route and get a dozen 6tb Western Digital Red drives.  In a Raid 6 (multiple drive failure) config you'd have 60tb of capacity.

For the amount of capacity you're looking for I don't see you spending less than $4,000 for any sort of reliability.  $6,000 is probably more likely as you'll need to build in some capacity for growth.

Have you done any sort of deduplication?